There's a (somewhat out of date) thread on the topic of beginning game development here and there's a (similarly somewhat out of date/incomplete) list of iOS game engines/libraries here.

Whilst it is possible to develop apps for iOS using Flash, be aware that Flash is apparently not as efficient as native code - consider that Machinarium was developed in Flash and, despite being a 2D game, requires a minimum of an iPad 2 - now consider that RAGE HD and Infinity Blade both require only a minimum of an iPhone 3GS... (Of course, depending on how complex your game is or isn't, Flash's efficiency - or lack thereof - may not be an issue...)